For a number of applications in which a power supply powers a connected device it is desirable for the power supply to provide a “power good” signal to the connected device to inform the device that the power supply is working correctly and is generating the correct voltage or voltages that the device requires in order to operate. This is because the power supply may take some time after initial start-up to establish proper operation. For example, the power supply may need to charge a line capacitance before it is capable of supplying a load. If the connected device were to begin operating before receiving the power good signal, the supply voltages could be incorrect, and this could lead to device malfunction. This is especially a problem where the device is a Power Over Ethernet Powered Device (POE PD) and connected to receive DC power over an Ethernet connection. By waiting for the power good signal from the supply before beginning to operate, this problem is largely eliminated.
It is known to supply power to a connected device by means of power supply rails incorporating a series connected vertical power MOSFET. The source and drain of the power MOSFET are connected in series with one of the rails, and by controlling the potential applied to the MOSFET gate the MOSFET can be switched between a conducting and a non-conducting state. Thus, the power MOSFET forms a controllable interrupt. For POE applications the potential between the power supply rails may be in excess of 30V, for example 48V, and in its off state the power MOSFET needs to withstand this voltage across its source and drain. A problem in such applications is how to generate a power good signal that is a good indicator of the power supply being ready to supply power to a load.